The present invention is directed to a polyaxial bone screw for use in spinal surgery and the like and especially to such a screw adapted to receive a rod member and secure the rod member to a vertebra or the like.
Many spinal surgery procedures require securing various implants to bone and especially to vertebrae along the spine. For example, elongate rods are often required that extend along the spine to provide support to vertebrae that have been damaged or weakened due to injury, disease or the like. Such rods must be supported by certain vertebra and support other vertebra. The most common mechanism for providing such structure is to implant bone screws into certain bones which then in turn support the rod or are supported by the rod. Bone screws of this type may have a fixed head relative to a shank thereof. In the fixed bone screws, the head cannot be moved relative to the shank and the rod must be favorably positioned in order for it to be placed within the head. This is sometimes very difficult or impossible to do so polyaxial bone screws are commonly used. The polyaxial bone screws allow rotation of the head about the shank until a desired rotational position is achieved for the head relative to the shank after which the rod can be inserted and the position of the head eventually locked with respect to movement relative to the shank.
The present invention is directed to such swivel head type bone screws and, in particular, to swivel head bone screws having an open head that allows placement of the rod member within the head and then subsequent closure by use of a closure top, plug or the like to capture the rod in the head of the screw.
Because such implants are for placement within the human body, it is always desirable for the implant to have as little effect on the body as possible. Consequently, it is quite desirable for the implants to have a relatively small profile both in height and width. It is also desirable that the implants be lightweight.
Furthermore, it is desirable that the swivel head implants be unlikely to unintentionally disassemble within the body. It is very undesirable for pieces of the implant to be free to move around within the body after surgery is completed and it also assures that the implant retains an ability to correct the structural problem for which it was implanted. Furthermore, if the implant should slip or become loose for some reason, it is still desirable for all of the parts to remain together and not separate.
Consequently, it is desirable for there to be a lightweight, low profile polyaxial bone screw which assembles in such a manner that each subsequent piece locks proceeding pieces within the overall structure, so that there is less likelihood that the various pieces of the structure will undesirably disassemble.
The present invention is directed to a polyaxial bone screw that comprises a shank, a head and a retainer ring that operably cooperate with each other. The bone screw is designed to allow the shank to be locked or secured in a selected angular configuration with respect to the head, while the head receives a rod member and while the shank is implanted in a bone, such as a vertebra or vertebral body.
The shank has an implant body which includes an external helically wound thread that is in turn attached by a neck to a capture end with a capture or connector type structure. The capture structure is positioned outside the bone in use and has a radiused and cylindrically shaped radially outer surface that has at least one radially outwardly extending non helically wound projection or spline thereon. The capture structure also has an upper axially aligned and radiused dome that protrudes above the remainder of the shank and above the ring during use to manipulate the shank and to contact the rod. Further, in some embodiments the shank includes off axis apertures, grooves, side slots or the like for use by an installation tool with a mating configured head for driving and rotating the shank into the bone.
The head has a generally cylindrical shaped profile with an upwardly open U-shaped channel formed therein so as to effectively produce a lower base with two upstanding and spaced arms. The inner surfaces of the arms have a threadform thereon or another suitable guide and advancement structure such as a helically wound flangeform for use in closing the upper part of the channel. Located in the interior of the base and coaxially aligned with the head is a chamber having an interiorly facing partial spherical shaped surface. The chamber further opens onto a bottom surface of the head through a head lower wall bore forming a constricted or restrictive neck sized and shaped to allow passage of the capture structure therethrough.
The retainer ring includes an external partial spherical or hemispherical surface that is sized and shaped to be seated in and slidably engage the partial spherical surface within the head, both having approximately the same radius of generation. The ring also has an internal, centrally located and axially extending ring bore sized and shaped to receive the capture structure of the shank therethrough. Further, the ring has a series of axially extending channels positioned about and opening into the central bore that are sized and shaped to allow sliding passage of the shank splines entirely through the ring so that the shank can be inserted through the ring while the ring is positioned within the chamber in the head. The channels are not helically wound about the bore and preferably extend vertically or parallel to the axis of the ring. The ring further includes a set of recesses that are circumferentially spaced from the channels and that open onto the upper part of the ring and into the bore, but do not pass entirely through the ring and that can be entered by the splines by drawing the shank with the splines thereon axially downwardly with respect to the ring. In this manner, the splines can be passed upwardly or uploaded through the ring by sliding through the channels in conjunction with the remainder of the shank capture and after sliding completely through the bore, the shank is then rotated a certain number of degrees relative to the head, and then drawn back downwardly or downloaded so that the splines encounter and engage the recesses wherein the splines are captured by the ring. The splines preferably have a wedge-shaped surface thereon which pushes not only downwardly, but radially outward against the retainer or capture ring when force is applied to the top of the shank.
During assembly, the ring is placed through the U-shaped channel into the chamber having the partial spherical surface and then rotated so that the ring hemispherical surface mates with and slidably engages the head partial spherical surface. Subsequently, the shank capture structure is uploaded into and extended through the ring central bore, while the splines pass through the channels. The shank is then rotated relative to the ring and then the shank is moved in an axially reverse direction opposed to uploading while the splines are positioned over the recesses so as to be aligned with the recesses and not aligned with the channels and so that the splines then enter the recesses. The ring with connected shank effectively thereafter form a ball and socket joint with the head and allow free rotation to a selected angular configuration until later locked in the selected configuration. The shank, head and ring are then placed in a bone by screwing the shank body into the bone using the apertures on the top of the shank or alternative structure such as grooves or faceted surfaces on the outside of the portion of the shank extending above the ring.
Thereafter, a rod is placed in the U-shaped channel and captured therein by closing the channel by use of a closure top or plug having a threadform or other external guide and advancement structure that mates with and advances along mating guide and advancement structure of the arms of the head, when the closure top is rotated. Preferably, the closure top also includes a break-off head that provides purchase for a tool for rotation and torquing of the closure top to a preselected torque and that such torque is transferred and applied as pressure against the rod received in the head. Once the preselected torque is achieved, the break-off head breaks away from the closure top. Under pressure from the closure top, the rod pushes against the dome of the shank that extends above the ring and thereby urges the splines downwardly. Because of the wedge shaped structure of the splines, the splines push both downwardly and outwardly upon the retainer or capture ring, when force is applied to the dome, so as to frictionally engage and positively seat the retaining ring in the cavity and prevent further rotation in conjunction with the shank dome frictionally engaging the rod under pressure. In particular, the hemispherical surface of the ring abuttingly and frictionally mates with the integral hemispherical interior facing surface of the head, while the dome frictionally mates with the rod under pressure from the rod so as to lock the shank and ring in a selected angular configuration relative to the head. The shank, in this manner, can be locked in a configuration selected from an infinite number of angular configurations with respect to the head. Once fully assembled in this manner, unless a part breaks into pieces, the shank cannot disengage from the capture ring and the head without disassembly of the device by reversing the process or breaking the parts.
Therefore, the objects in the present invention are: to provide a polyaxial bone screw having a bone implantable shank that can be locked in a fixed position relative to a head of the bone screw; to provide such a bone screw having a capture or retaining ring having a partial external hemispherical surface that seats within a partial internal spherical shaped chamber surface within and integrally formed with a head of the bone screw to form a ball and socket joint and wherein the shank is securable to the retaining ring; to provide such a bone screw wherein the shank has at least one spline that extends radially outward from a capture end thereof and wherein the retaining ring has a central bore that receives the capture end while a channel opening into the bore allows the spline to slide through the ring so as to pass above the ring, at which time the shank can be rotated a select number of degrees and further wherein the retaining ring has a capture recess that receives the spline on further downward or reverse movement along the axis of the shank relative to the retainer ring; to provide such a bone screw wherein the shank has an upwardly protruding radiused dome which has a radius that in one embodiment is substantially less than the radius of the external hemispherical surface on the retainer ring and that extends upwardly within the head chamber so as to reduce height of the screw head and further, is operably positioned so as to engage a rod member received in the head so as to receive downward pressure from the rod during assembly; to provide such a bone screw wherein a closure top is used to close a channel in the bone screw head after receiving the rod and to apply pressure to the rod member that in turn, exerts pressure on the dome of the shank so as to urge each spline into an abutting and tight relationship with the retaining ring and to urge the retaining ring both downwardly and radially outwardly, so that the external hemispherical surface on the retaining ring more completely contacts and frictionally engages the internal spherical surface within the chamber of the head thereby providing improved mechanical fixation to prevent further rotation of the shank relative to the head; to provide such a bone screw which has a comparatively low profile and which is comparatively light in weight; to provide such a bone screw that resists disassembly thereof except by removal of the closure top; to provide such a bone screw wherein the closure top can be removed by the surgeon should disassembly be desired at which time the entire structure can be easily and quickly disassembled and removed from the bone, if necessary; to provide such a bone screw wherein the shank is locked in position relative to the head during usage in a locked or fixed configuration; to provide such a bone screw wherein the shank cannot disassemble from the head and the retaining ring once the screw is fully assembled except if the closure top is removed and the device is disassembled by the surgeon; to provide such a bone screw which is easy to use and extremely effective for the intended usage thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.